AN4076

Current sensing circuit design guidelines

Doc ID 022923 Rev 1

5/14

Figure 3.

Motor current feedback conditioning circuit

1.1

Shunt resistors dimensioning

The starting point for the design of the current sensing network is the dimensioning of the

current, and therefore the available useful signal. On the other hand, power dissipation on

the shunt resistor increases together with its resistance value, so the value of resistance

component.

It is therefore important to know how to calculate the power dissipation on the shunt resistor

as a function of the expected maximum motor current (that is the motor rated phase

current).

In fact, it must be noted that the motor phase current really flows through the shunt resistor

only if the low-side transistor of the same leg is turned on. Fortunately, if we consider a full

motor phase current electrical period, the average duty cycle applied to the low-side switch

is always 50% in ideal conditions (constant bus voltage, constant torque and speed,

sinusoidal motor B-emf, …), independently from the modulation index which is used for the

space vector modulation (SV-PWM) and which fixes the portion of bus voltage really applied

to the motor phases.

Additionally, as shown in Figure 4, the SV-PWM duty cycle applied at angle

θ+Π is

complementary to the duty cycle applied at

θ. For this reason the integral

may be graphically computed (see red lines of Figure 4) as

AM12078v1

Rshunt

+

-

U1A

3

1

2

R1

C1

R2

R3

V_m

R4

R5.

Filter

Offset

C2

Amplification

To MCU ADC pin

Low side switch

High side switch

To motor phase

Vbus

i

2

R

0

T

∫

t

()dt

i

2

S

0

T

∫

t

()dt

1

2

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